In a general manner, the respiratory pathology known as “Sleep Apnea Syndrome” (SAS) is characterized by the frequent occurrence (at least 10 to 20 times per hour) of apneae during a sleep phase of the patient. An “apnea” (or respiratory pause) is defined as a temporary stop of the respiratory function, with a duration longer than 10 seconds. SAS can also be characterized by the occurrence of hypopneae under the same conditions. A “hypopnea” is defined as a significant decrease (but with no interruption) of breathing airflow, typically a decrease of more than 50% as compares to an average of preceding airflow.
Facing this pathology, what affects more than 4% of the population, and more than 50% of the patients suffering from heart failure, the autonomic nervous system becomes adapted, but with a noxious effect on sleep, to the interruption or reduction of breathing airflow leading to a decrease of the blood oxygen concentration, as well as unconscious micro-awakenings. That is followed, during arousal, by diurnal sleepiness with a loss of attention and increased risks of road accidents. Moreover, the physiologic, then pathologic, adaptive response of certain organs including the heart and respiratory system leads to a greater incidence of such disorders as arterial hypertension, ventricular arrhythmiae, myocardial infarction and heart failure.
Diverse techniques intended to detect sleep respiratory disorders by means of an implantable device are known in the prior art.
European patent EP 0970713 and its U.S. patent counterpart U.S. Pat. No. 6,574,507 (commonly assigned herewith to ELA Medical) discloses a device that diagnoses the occurrence of an apnea based upon a signal representing minute ventilation (VE signal, or MV signal), a preponderantly physiological parameter, usually obtained through a measurement of transthoracic impedance, providing a continuous indication of the patient's respiratory rhythm.
One drawback of this technique relates to the fact it requires that the patient be implanted with a device, which is a limit to the usefulness and effectiveness of this technique.